change how thumbnails are generated

Hello. I need to change how thumbnails are generated for “related content.” Right now it appears the thumbnail is generated by using the center of the uploaded feature image. I need thumbnails to be generated from the left side of the image. This is because I make an image that has the post image on the left, then a heading and logo on the right.

While the obvious fix is to just not use the heading, I like it and want to keep it. So, I need to understand how thumbnails are generated, then where and how to change things so the thumbnail generates from the left side of the feature image upload, instead of the center.

You can see what I am talking about at this post: https://www.flyingc-diy.com/2014/04/01/perking-saggy-furniture-cushions/

Thanks a bunch!
Cori